The 2010-2016 nuclear security summit process focused leaders’ attention on nuclear security and preventing nuclear terrorism. Momentum has waned since the final summit in 2016. Despite continuing nuclear security improvements in some countries, and ongoing efforts to maintain or strengthen the international nuclear security framework, serious vulnerabilities still exist in nuclear security systems around the world. Many nuclear facilities are not protected against all of the plausible threats adversaries might pose—especially in the case of threats from insiders. Nuclear materials remain unnecessarily vulnerable in too many locations. The culture within many nuclear organizations is insufficiently focused on security. Many nuclear security systems are not being exposed to sufficiently in-depth, creative, and realistic vulnerability assessments and testing. This paper will summarize the post-summit evolution of nuclear security efforts and offer recommendations for regaining international nuclear security momentum through combating complacency, improving implementation on the ground, strengthening frameworks for international cooperation, and maintaining nuclear security leadership.